1. Field of the Invention
The present invention relates to an image forming apparatus including a charge device configured to charge a surface of a photosensitive member by discharging static electricity.
2. Description of the Related Art
An electrophotographic image forming apparatus is designed to form an image on a recording medium by an electrophotographic image forming process. Examples of the image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a color laser beam printer and a color LED printer), a multifunction printer (MFP), a facsimile machine, and a word processor. The image forming apparatus represents a color image forming apparatus as well as an image forming apparatus for forming a monochrome image.
The image forming apparatus includes a photosensitive member. Examples of the photosensitive member include a drum-shaped or belt-shaped photosensitive member having a photoconductor as a photosensitive layer. The photosensitive layer is made of such a material as amorphous selenium, zinc oxide, cadmium sulfide, amorphous silicon, or an organic photoconductive material.
In an electrophotographic process of the image forming apparatus, first, the charge device charges the surface of the photosensitive member uniformly. Then, an exposure device emits light onto the uniformly-charged surface of the photosensitive member according to image information to form an electrostatic latent image on the surface of the photosensitive member. A developing device causes developer (toner) to adhere to the electrostatic latent image to obtain a toner image. A transfer device transfers the toner image from the photosensitive member to the recording medium. A fixing device fixes the toner image onto the recording medium. The recording medium having the image formed thereon is delivered to a delivery tray.
As one of the charge devices for the image forming apparatus, there is a corona charging device. The corona charging device charges the surface of the photosensitive member by corona discharge. The charge device includes a shield casing having an opening portion opposed to the surface of the photosensitive member, a discharge wire arranged inside the shield casing, and a high-voltage power supply for applying a high voltage to the discharge wire. The discharge wire is a metal wire having a diameter of about 50 to 100 microns (μm). The high-voltage power supply applies to the discharge wire a high voltage of about 5 to 10 kilovolts (kV) to cause corona discharge around the discharge wire. Through the corona discharge, air around the discharge wire is ionized to generate ions. The ions are supplied to the surface of the photosensitive member so that the surface of the photosensitive member is charged.
To the discharge wire, foreign substances such as silicon compounds may adhere, resulting in uneven charge. Therefore, the discharge wire needs to be cleaned or replaced on a periodic basis.
Further, the photosensitive member deteriorates due to ozone generated by corona discharge. The photosensitive member has, a characteristic that the surface thereof is likely to absorb moisture increasingly along with the deterioration process of the photosensitive member due to corona discharge. Ozone reacts with moisture in the air to generate an ozone product, which adheres to the surface of the photosensitive member that is likely to absorb moisture, The ozone product causes a drop in surface resistance of the photosensitive member to hinder the sufficient charge of the photosensitive member when the electrostatic latent image is formed, with the result that image deletion occurs. There is a technology for preventing the image deletion by constantly heating the photosensitive member by a heater to remove moisture from the surface of the photosensitive member (Japanese Utility Model Publication No. H01-34205). However, the ozone product is generated to a great extent during, tor example, nighttime in which the image forming apparatus is not in use, Therefore, the photosensitive member needs to be heated constantly by a heater, resulting in higher power consumption.
In view of the above, there is a technology for keeping an ozone product generated hear the discharge wire away from the photosensitive member by arranging a shield member between the photosensitive member and the charge device (Japanese Patent Application Laid-Open No. 2007-072212). Japanese Patent Application Laid-Open No. 2007-072212 discloses that the heater for warming the photosensitive member is turned of in a power saving mode and, at the same time, the shield member is moved so that the shield member photosensitive member forms the charge device. Because the heater can be turned off, the power consumption an be reduced.
In the corona charging device, the clearance between the charge device and the photosensitive member is set as small as about several hundred μm to 2 millimeters (mm). In this structure, the shield member having a thickness of about several dozen μm may be moved through such a small clearance. If the photosensitive member is rotated when the shield member is situated at a closed position at which the charge device is isolated by the shield member, however, the shield member may interfere with the photosensitive member due to wind pressure and vibration caused by the rotation of the photosensitive member, and the shield member may consequently damage the photosensitive member. The damage to the photosensitive member may cause an image defect. Further, the damage to the photosensitive member may cause leakage of the high voltage discharged from the charge device. The leakage of the high voltage may result in malfunction of the image forming apparatus.